1. Field of the Invention
The present invention relates to drive devices for controlling the operation of an electric power conversion circuit (or an inverter) connected to an electric rotary machine such as a motor generator.
2. Description of the Related Art
Electric power conversion circuits (or inverters) are well known. In general, such an electric conversion circuit is arranged between a direct current power source and an electric rotary machine. The electric power conversion circuit connects terminals of the direct current power source with terminals of the electric rotary machine. In general, the electric power conversion circuit is comprised of switching elements in a high voltage side and switching elements in a low voltage side. For example, power switching elements such as insulation gate bipolar transistor (IGBT) are used as these switching elements. The electric power conversion circuit is composed of some pairs of these switching elements. Each pair is composed of the switching element in the high voltage side and the switching element in the low voltage side connected in series.
Each of the switching elements in at least one of the high voltage side and the low voltage side in the electric power conversion circuit is equipped with a freewheel diode. In particular, the switching element and the corresponding freewheel diode are connected in parallel.
In order to generate and then flow a current having a sine-curved waveform in an electric rotary machine, it is necessary to drive the switching elements in the high voltage side and the switching elements in the low voltage side so that the switching elements in the high voltage side and the switching elements in the low voltage side are alternately driven. This makes it possible to drive the pairs of the switching elements in the high voltage side and the switching elements in the low voltage side in a complementary manner.
On the other hand, in general, each of the switching elements used in the above electric power conversion circuit, namely, the inverter is made of an insulation gate bipolar transistor (IGBT). Recent trend uses an IGBT with a freewheel diode. In particular, the IGBT and the freewheel diode are connected in parallel on the same semiconductor substrate.
Because a current flowing from the collector to the emitter is a forward current in the IGBT, no reversed current of the forward current flows in the IGBT. However, when the pairs of the switching elements in the inverter are driven in a complementary manner, there is a possibility of no current flowing in the switching element which is turned on according to a direction of the current of a sine curved waveform. This case further causes for the freewheel diode connected in parallel to the switching element to enter a freewheel mode. In the freewheel mode, a current flows in the freewheel diode where the switching element and the freewheel diode are reversely connected in parallel with each other.
It is known that the amount of a voltage drop in the above power switching element, namely, the IGBT with the freewheel diode is increased by supplying a voltage to the gate terminal of the IGBT when a forward current flows in the freewheel diode. This increases the electrical power loss generated in the freewheel diode, and further increases the entire electrical power loss of the IGBT with the freewheel diode.
In order to avoid such a conventional problem, Japanese patent laid open publication No. JP 2008-72848 has disclosed a conventional technique in which a control device forcedly turns off the freewheel diode when a current flowing in the freewheel diode is detected even if receiving an instruction signal to turn on a pair of IGBTs with a freewheel diode in a complementary manner. Specifically, small-sized electrodes are formed on the same semiconductor substrate on which the freewheel diode built-in the IGBT is formed, and the control device detects a small current through the small-sized electrodes. For example, such a small current is within a range of one-several thousandth to one ten-thousandth of a current actually flowing in the freewheel diode. When detecting the small current flowing through the freewheel diode through the small-sized electrodes, the control device forcedly turns off the switching element such as an IGBT. This can suppress the electrical power loss of the IGBT with the freewheel diode from increasing.
However, it is in general difficult to increase the detection accuracy to detect an actual current flowing in the freewheel diode on the basis of such a small current detected through the small-sized electrodes because such a small current value is very small. In other words, it is difficult to detect the current value with high accuracy and to turn off the switching element such as the IGBT equipped with the freewheel diode on the basis of the detection result of a small current through the small-sized electrodes unless a large current flows in the freewheel diode.
A control system using other types of switching elements with a built-in freewheel diode, other than such IGBT with a built-in freewheel diode, also have the same difficulty to timely inhibit the turning-on operation of the switching element when the freewheel diode is in the freewheel mode where a forward current flows in the freewheel diode.